With electromagnetic actuators that are used for actuating a control element, in particular a gas exchange valve on an internal combustion engine, recording the armature movement allows drawing conclusions about the actual movement conditions and, based on this, influencing the armature movement and thus also the movement of the control element to be actuated via the current supplied to the electromagnets of the actuators for the purpose of automatic control.
An electro-inductive sensor is used to record the movement, which sensor essentially consists of a fixed coil arrangement and a bar-shaped sensor piece that can be moved relative thereto, the bar-shaped sensor piece being connected to the control element and provided with a so-called eddy current ring, or short-circuit ring. The coil arrangement, which consists of at least two coils that are positioned successively in the axial direction, is supplied with a high-frequency alternating current, so that movements of the short-circuit ring relative to the coils change the electrical quality of the coils as a result of the opposing magnetic field generated in the short-circuit ring. The movement can be derived in the form of a signal from this movement-dependent change.
It is critical for the accuracy of this sensor technology that interfering voltages that are present in the system or are generated by the system itself are prevented, or at least are reduced enough, so that they do not interfere with the generated signal.
With an electromagnetic actuator for actuating a gas exchange valve in an internal combustion engine, having an armature that moves back and forth against the force of restoring springs between the pole faces of an opening magnet and a closing magnet that face each other at a distance, the armature in the respective end positions comes to rest against one of the pole faces. The arrangement in this case is such that a separate spring bolt is associated with the armature on the side of the armature adjacent to the closing magnet, the spring bolt operating jointly with the opening spring and supporting itself loosely, but being frictionally-connected on the armature.
The opening movement is initiated through a mechanical excitation of the bar-shaped sensor piece, also called a measuring stilt, which is rigidly connected to the spring bolt, as a result of the existing valve play between the gas exchange valve in the closing direction and the armature that rests against the closing magnet. This mechanical impact stress generates high-frequency structure-borne sound waves within the material of the spring bolt and the bar-shaped sensor piece connected thereto, which are then reflected at the ends and consequently traverse back and forth and cause longitudinal oscillations. These longitudinal oscillations interfere with the movement of the bar-shaped sensor piece and its short-circuit ring, leading to a corresponding interference that changes the coil quality, which in turn results in a high-frequency interference voltage. This voltage cannot be suppressed with the normally used evaluation device in the form of a carrier frequency measuring bridge, since the operating frequencies of the frequency measuring bridge and those of the interfering voltage are close together, thus making it impossible to obtain a usable signal.
Increasing the wattage of the feed voltage by a factor of 100 would only result in a gain of 20 dB with respect to the signal-to-noise ratio.